Spoiler alert - just in case you live under a rock and havn't seen "Star Wars - The Force Awakens"
I have seen several theories about the reason why Kylo Ren was so easily defeated by Flynn and Rey

Here is another thought about Chewbacca’s Bowcaster.

Could the Bowcaster be a Force weapon?

Every character struck by this weapon was blown away, except those very powerful in the use of the Force. Each person struck was hit with the force needed to strike them down. Only the most powerful Sith were able to deflect it in any way, or survive getting struck by one if it’s bolts.

 
It has been said that those chosen for becoming Stormtroopers, or for cloning, were force sensitive making them easier for Palatine to control while leading a battle from his throne. 

As Chewie fired the bowcaster it seems that it adjusts its strength to the level of Force control of the target. But it had its limits as shown by Darth Vader being able to avoid it completely.

Kylo Ren, although not able to fully deflect the bolt due to his mental state after killing Han Solo, was able to withstand the bolt and hold himself together. Any other person Sith or Stormtrooper would have been killed. Kylo was struck by the first bolt, held himself together and was able to avoid further shots from it. He was obviously drained by the injury. Could the bolt also have that capability? Able to partially neutralize the Force in someone struck by its bolt?

This would explain why spending so much of his energy holding himself together would make him unable to fully defeat Rey and Flynn.

Remember Flynn was a fully trained Storm Trooper, most likely trained and highly skilled in hand to hand weaponry and at first Kylo was able to easily handle him at the start of their fight. But not Rey after fighting Flynn, with Kylo’s Force strength ebbing due to the bowcaster effect. 

Not just the physical injuries, but His Force capabilities continued to weaken from the Force draining effects of the Bowcaster.

Anyway... That is my Bowcaster Theory.